GW1. They were a PITA. Constantly going off in the presence of MT fumes (oils, exhausts etc) so units sensibly reached the decision that unless you heard/saw an explosion or were attacked, you were pretty safe and could leave them switched off. Unless your really, really liked full IPE.

when we trained with them we used fly spray up wind at a few hundred meters. if they broke they would stick them on a shelf for a year then send you them back - must have had self healing circuitry or something.

We called that the M8 Chemical Agent Alarm System. I took NBC Training in 1982 and I remember having to dick around with one of these things in training. Very fussy and prone to false alarms. They were used extensively in GW1 by us but not so much in GW2. (Invasion of Iraq 2003) I forgot all the details about its operation but if I remember right it sniffed the air and if there was something in it that reacted with the chemical in the box it sounded the alarm. Skeeter repellent could do it sometimes.

NAIAD (Nerve Agent Immobiliser and Agent Detector) supposedly did what it says on the tin and detected nerve agents. Nerve agents are chemicals that interfere with an enzyme found in the tiny gaps between the bits of your nervous system, called acetylcholinesterase. The mechanism of nerve agent action is moot, just remember that it basically stops this acetylcholinesterase from working and you PRIT-HIDE-NIMS to oblivion.

Right, NAIAD had a pad that contained butyrylcholinesterase, which for the purposes of the detector, is affected by nerve agents in the same way as acetylcholinesterase is. So far, so simple(ish).

Now this pad, which is exposed to the ambient environment (and so any possible agent), is continually washed with a special solution that is affected by the butyrylcholinesterase. The butyrylcholinesterase helps a reaction take place in the special solution, that changes the electrical properties of this special solution. The run off from this solution goes into a container where some electrodes monitor its electrical potential.

All being well, there is no nerve agent to stop the butyrylcholinesterase from working, so it can affect the special solution, leading to a 'routine' set of electrical properties of the solution. If the butyrylcholinesterase is affected by nerve agent (or, annoyingly, quite a few other things) it doesn't work as well on the special solution, so the electrical properties of the runoff change. This is sensed and it alarms. You mask in 9 and, err, then discover it's gone off by mistake.

All of the above - and details of the exact chemicals, etc - can be found Open Source if you know where to look....